Nowadays, ethylene oxide is produced by catalytic gas phase oxidation of ethylene using a molecular oxygen-containing gas in the presence of a silver catalyst. An outline of a purifying method in a process for producing ethylene oxide is as follows (for example, refer to JP 62-103072 A).
First, ethylene and a molecular oxygen-containing gas are subjected to catalytic gas phase oxidation on a silver catalyst to obtain an ethylene oxide-containing reaction product gas (reaction step). Subsequently, the resulting reaction product gas is introduced into an ethylene oxide absorption column. The reaction product gas is brought into contact with an absorption liquid mainly containing water. Ethylene oxide is recovered as an aqueous solution (absorption step). Subsequently, the recovered ethylene oxide aqueous solution is fed to a purification system of ethylene oxide to obtain high-purity ethylene oxide through several stages. The ethylene oxide purification system usually includes a stripping step, a purification step, a dehydration step, a light fraction separation step, a heavy fraction separation step, and the like.
Usually, an exhaust gas containing unreacted ethylene discharged from a column top part of the ethylene oxide absorption column, a carbon dioxide gas (carbon dioxide; CO2) and water as by-products, and an inert gas (nitrogen, argon, methane, ethane, or the like) is circulated into an ethylene oxidation step as it is. Alternatively, apart thereof is extracted and introduced into a carbon dioxide gas absorption column, and the carbon dioxide gas is selectively absorbed by an alkali absorption liquid. The absorption liquid is supplied to a carbon dioxide gas stripper column to strip and recover the carbon dioxide gas (for example, refer to JP 60-131817 A).